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<h1><b>Subclassing</b></h1><p>
<ul><li><h2>Classes are designed and documented for subclassing.</h2>
<li><h2>Subclasses cannot relax constraints</h2>
<li><h2>Subclasses can add constraints through protected check methods</h2>
<li><h2>Subclasses can add functionality or utility</h2>
<li><h2>Factories can be used to build in the subclasses during parsing</h2>
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